Coal gasification has become a subject of many studies in recent years as a conversion technique which shows promise of providing a substitute for high BTU natural gas as reserves of petroleum and natural gas diminish. The cost of this substitute natural gas, better known as synthetic natural gas (SNG), may be several times the cost of natural gas which has existed in the past. The major cost of surface coal gasification consists of coal mining cost and cost of the gasification plant. Mining cost increases dramatically when the ratio of the overburden to coal seam thickness increases as in the case of strip mining and as one goes deeper to mine coal by conventional deep mining methods. Also, various environmental and health related problems occur such as surface disruption, health and safety of miners, sulfur removal, pollution from fly ash, waste disposal and others.
Compared to conventional mining of coal combined with surface gasification, underground coal gasification offers a number of significant potential advantages such as:
It can be applied to large reserves not economically recoverable by conventional mining methods; PA1 It minimizes, health and safety problems associated with conventional coal extraction techniques since no mining is required; PA1 It can produce less surface disruption and brings less solid waste to the surface; PA1 It consumes less water and generates less atmospheric pollution; PA1 It reduces socioeconomic impact; PA1 It reduces capital investment and gas cost.
The possibility of underground gasification of slack and waste coal in mines was first suggested by William Siemens in 1868 (Siemens W., Transactions of Chemical Society, 21, 279, 1868). In 1888, Mendeleev in Russia suggested the true underground gasification of coal. In 1909, A British Patent (G.G. 21674) was granted to Ansen Betts who proposed a method of gasifying coal in the bed by igniting the base of one or more shafts or boreholes, supplying it with air and steam and withdrawing the gas formed through either the same or other shafts or boreholes. Small-scale experiments on underground gasification were conducted in England by Sir William Ramsey prior to the first World War. The Russian work, inspired in part by Lenin's recommendations, was of the largest duration, beginning (on a large scale) in about 1933 and extending to about 1965. It reached the state where the extracted gas was used for large-scale generation of electricity and to supply local industries (see Report No. UCRL-52004 and SAND 76-0380 available from NTIS, Springfield, Va.). Outside Russia most of the activity occurred in the post World War II period from about 1945 to 1960. At one time during this period, simultaneous large-scale experimental work was underway in England, France, Belgium, the United States, Italy and continuing work in USSR; and probably in Poland, Czechoslovakia and Japan. (See Chapter 21, Chemistry of Coal Utilization; Supplementary Volume, by H. H. Lowry, John Willey & Sons, New York). Some of the operations were claimed to be promising but availability of lower cost petroleum and natural gas prevented further development. In all cases, work appears to have ceased mainly for lack of economic incentives, until recently when shortage of petroleum and natural gas became prominent and the price of oil and natural gas escalated significantly. Renewed interest in the last few years has again stimulated research in underground coal gasification area.
Underground coal gasification involves two basic steps: (1) preparation of the coal seam and (2) gasification of the coal. The majority of the emphasis is being placed on the first step, i.e., to prepare the coal seam to improve the permeability of coal by development of a variety of fracturing methods, novel drilling techniques and by development of the configuration of boreholes, etc.
Very little emphasis has been placed on the second step, i.e., in gasification process tactics inside the coal seam. Most of the processes of the prior art have been based on air injection or air and steam injection where the product gas Btu content is very low. However, little attention has been given to the chemistry of coal gasification inside the coal seam and on improvement of the heat content of the product gas. U.S. Pat. No. 3,734,184 to Scott has suggested a method for distilling coal in situ by treating a rubblized bed of coal with super heated steam to recover hydrocarbons and to improve the Btu content of the product. U.S. Pat. No. 3,794,116 to Higgins has suggested use of oxygen and steam instead of air to improve the Btu content of the product gas.